1. Field of the Invention
This invention relates to a hollow valve for internal combustion engines. In particular, the invention relates to a hollow valve that is capable of reducing the temperature of the valve and is lighter in weight.
2. The Prior Art
Hollow valves represent a particular type of design of outlet valves that are subject to higher stress. Such valves are used for lowering the temperature in the region of the hollow throat and the valve disk, but also to reduce the weight of the valve. When hollow valves are used for lowering the temperature, the hollow space is filled with metallic sodium up to about 60% of its volume. The sodium transports the heat from the head to the stem of the valve, where it is t hen discharged via the valve guide.
Several hollow valve variations are known. With one of such variations, the basic body is drilled hollow and then joined with a massive end piece of the shaft by friction welding. The end piece of the shaft can be hardened and therefore permits stabilization that is adapted to stress.
Another variation is substantially more expensive to manufacture. This basic body is drilled from the end of the shaft as well. However, the bore is sealed by inductive heating of the end of the shaft with subsequent closing of the bore by forging. The massive end piece of the shaft is attached by friction welding. Friction welding of the massive end of the shaft with the closure of the end of the basic body offers higher safety. This type of design is used predominantly in high performance engines.
The optimum weight reduction and heat discharge from the surface of the valve disk has previously been achieved by a hollow head valve. In contrast to the above mentioned designs, such a valve is drilled and worked from the disk side. The aperture is sealed by inserting a lid or cover. The application of such valves is predominantly limited to the field of motor sports, as their complicated manufacture leads to high costs. To avoid subjecting the valve shaft seals to excessive temperatures, however, the bore of the valve may extend only up to the proximity of the sealing lip of the elastomeric valve shaft seal extending on the outer diameter. A longer bore can be drilled only if no valve shaft seal is used.
Hollow valves may be manufactured as mono-metal valves. However, bimetal valves are more common. In these valves, the stem consists of a ferritic-martensitic steel and the head consists of austenitic material or a nickel-based material. This material is shaped hot as the head piece is being manufactured before it is drilled hollow.
European Patent No. EP 0 619 419 B1 discloses that the manufacture of very light valves is known. This type of valve is manufactured as a single piece, hollow cylindrical component from a ductile metal sheet material. These valves, however, are mono-metal valves that can only be used in lower stress environments, because austenitic material has heretofore been excluded from cold-forming processes. Furthermore, in the known valve having a deep-drawn valve body, the hollow space extends up to the outermost end of the shaft. The discharge of heat through a valve filling, if any is used into the end piece of the stem, however, could be damaging under certain circumstances. The use of such valves for lowering the temperature is very questionable, especially with valves having valve guides with a terminal sealing of the valve shaft.